A British security guard was among two crew members killed in what a UK source described as a suspected drone attack on an Israeli-linked oil tanker off Oman.
There has not been any official confirmation about what happened to the Mercer Street vessel, which is managed by a British company, and no one has claimed responsibility.
The UK source said that the crew had reported being targeted by "some sort of drone" on Thursday in the Arabian Sea before communications with the ship were lost.
If a drone attack is confirmed it would raise speculation about a possible link to a government or some kind of proxy group.
Iran in the past has repeatedly been accused of targeting tankers in the Gulf.
The country has been locked in a tit-for-tat contest with arch-rival Israel, with both sides accusing the other of targeting ships and launching cyber attacks.
Zodiac Maritime, the UK firm that manages the vessel, is part of Israeli billionaire Eyal Ofer's Zodiac Group.
It said "details of the incident are still being established and an investigation … is currently underway".
The tanker was on the move again, under the control of its crew, with a US naval escort to a safe location, Zodiac Maritime added, without specifying where it was headed.
The British national who was killed was working for the UK maritime security company Ambrey. The other fatality was a Romanian crew member.
A British government spokesman said: Our thoughts are with the loved ones of a British National who has died…
Vessels must be allowed to navigate freely in accordance with international law. We are working with our international partners to urgently establish the facts."
In a sign of the gravity of the situation, it is understood that Dominic Raab, the foreign secretary, is receiving regular updates.
The United Kingdom Maritime Trade Operations (UK MTO) - a part of the Ministry of Defence, which provides maritime security information - said what took place was a "non-piracy incident".
The Mercer Street tanker is Japanese owned but was sailing with a Liberian flag.
Concerns about a threat to tankers from Iran, following a spate of attacks in 2019, prompted the UK and its allies to send more warships to the Gulf to better protect the vital waterway.
The UK MTO said the vessel was about 152 nautical miles (280 km) northeast of the Omani port of Duqm when it was attacked on Thursday.
It gave no more details on the type of vessel, any cargo, to whom it belonged or about possible casualties.
Zodiac Maritime said in a statement: "We can confirm that there has been a suspected piracy incident onboard the product tanker M/T Mercer Street.
"The ship is a Japanese owned vessel managed by our UK based Zodiac Maritime Office. At the time of the incident, the vessel was in the northern Indian Ocean, travelling from Dar es Salaam to Fujairah with no cargo on board.
"We are in coordination and liaising with the UK MTO and other relevant authorities."
A spokesperson for the Ministry of Defence said: "We are aware of reports of an attack on a merchant vessel off the coast of Oman. UK military headquarters in the region are currently conducting investigations."
According to Eikon's ship tracking, the Mercer Street was headed to Fujairah, a bunkering port and oil terminal in the United Arab Emirates, from Dar es Salaam, in Tanzania.
https://news.sky.com/story/briton-among-two-killed-in-attack-on-tanker-in-arabian-sea-12368065
